Embodiments of the invention generally relate to methods and apparatus for measuring the crystal fraction of crystalline silicon casted mono wafers. More specifically, embodiments of the invention are directed to methods and apparatus for measuring the crystal fraction of the wafer by one or more of transmitting near infrared light through the wafer and reflecting light off of the surface of the wafer and evaluating the determining the percent mono-crystalline surface area.
The two main types of bulk material used in solar cell technologies are monocrystalline and polycrystalline silicon. Monocrystalline wafers are relatively expensive, but produce more efficient solar cells. On the other end, polycrystalline wafers are cheaper to produce, but result in lower efficiency cells. Recently, a new technology named Casted-Mono (AKA Quasi-Mono) was introduced and has quickly gained traction. Casted-Mono produces solar wafers that are both efficient and low cost to manufacture.
The new process for casted-mono, suffers from high variation in quality, or more specifically-variation in mono crystal area of the wafers. Quality distribution of the crystal fraction (CF) correlates with high distribution of the expected efficiency of the solar cells to be manufactured out of these wafers.
Therefore, it is important to develop a method of inspecting the as-cut wafers and determine the crystal fraction. This information can be used to sort and grade the manufactured wafers and can also provide real time feedback on the manufacturing process. Currently there are no commercial inspection systems to measure the CF, and feedback on wafers' quality can be obtained only after the cell is 100% manufactured. Therefore, there is a need in the art for methods and systems to accurately and repeatedly measure the crystal fraction.